This invention constitutes a major improvement in electric actuators heretofore used for the driving of valves and other devices following the command signal of an automatic controller or computing device. A typical example for prior state of the art device is found in U.S. Pat. No. 3,150,752 by Baumann. Here a stepping motor converts electrical impulses into small turns of a lead screw which in turn drives a reciprocating valve stem up or down. With either power or signal failure, the actuator will remain in the last position of the lead screw, a condition that does not satisfy requirements of most automatic control valve applications.
A fuel control valve to a boiler should fail-close, in order to avoid a possible overheating, if electrical failure occurs. Certain coolant control valves, on the other hand, should fail-open.
There is, therefore, a great need for so-called fail-safe electric valve actuators to replace the currently used pneumatically actuated and fail-safe spring-diaphragm actuators. Presently used electric actuators employ gear drives or threaded spindles to convert the relatively high-speed electric motor revolutions into higher forces but slower output motion. It is inherent in these high mechanically amplified devices that their efficiency is less than 30%, negating any possibility of reversing the motion by spring means to achieve the desired "fail-safe" action. Other attempts have been made to drive the motor into a safe position upon an electric line failure by means of a relay switched battery. However, such a solution is not only awkward and space consuming but offers only limited reliability unless the battery is maintained periodically.
Operators using gear trains as mechanical amplification of forces suffer from wear and, more importantly, back-lash which impedes the desired accuracy of the actuator.
This invention overcomes these and other disadvantages of the current state of the art devices. Instead of gear speed reduction, the invention uses a double-pitched cable drive, which not only offers no back-lash caused by play between meshing gear teeth, but achieves great mechanical amplification with up to 80% efficiency. Such high efficiency in turn allows the use of mechanical springs to reverse the rotation of the electric motor drive in order to achieve a desired safety position following a power failure.
Connecting the spring-loaded actuating stem directly onto a valve shaft, eliminates problems of conventional mechanical override mechanisms to absorb thermal expansion of a valve shaft, featured, for example, in U.S. Pat. No. 3,150,752. The absence of gears and mechanical override devices leads to a dramatic simplification, great cost savings and a substantial increase in reliability over present similar devices.
An amplifying circuit is further designed to drive the motor only in a direction opposite to the direction of the spring force. Upon reaching the desired valve position, the motor drive current is replaced by a locking DC current. For reverse action, the DC current is switched off, allowing the spring force to drive the motor backwards. This "One-Way" switching action greatly simplifies the electronic control circuit, leading to important cost savings and increase in reliability.
Another important object of the invention is the provision of electronic means to limit the actuator travel whenever the feed-back voltage as function of the travel reaches an adjustable value of the input signal voltage, which then blocks all further stepping power to the actuator motor. This prevents the motor from reaching a stalled position and prevents it from going out of control. All this is accomplished with electronic means and without resorting to cumbersome mechanical switching means such as utilized in U.S. Pat. No. 4,097,786, for example.